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Chemical exergy of hydrocarbon fuels is traditionally released through direct combustion and is utilized as a form of thermal exergy. Consequently, the higher energy level of hydrocarbon fuels Af is immediately degraded to the energy level of thermal energy Ath, resulting in greater exergy destruction in fuel combustion, (Af - Ath) in Case 1.
Alternatively, the chemical exergy difference between Af and Asyn is used to convert methanol fuel to syngas first, followed by syngas combustion (fuel indirect combustion) where the chemical exergy Asyn is released to the thermal energy level of Ath. Obviously the energy level degradation from chemical energy to thermal energy is significantly reduced, (Asyn - Ath) in Case 2.
According to Figure 3, it is also noted that both fuel energy Qf and steam energy Qstm are added to the GT combustor for Case 2. Due to cascade utilization of fuel chemical exergy, the steam energy of Qstm at lower grade Astm is upgraded to the higher energy level Asyn. The ratio of Qstm to Qf is about 20 percent for Case 2, which leads to reduction of fuel consumption, resulting in significant efficiency improvement.
3 Pre-combustion CO2 capture combined cycle with methanol fuel
3.1 Pre-combustion CO2 capture combined cycle
Figure 4 shows a flow schematic of a pre-combustion CO2 capture combined cycle based on methanol fuel. Similar to Case 2, the liquid methanol at the pressure of more than 22 bar is first heated and converted into gas. The methanol gas is heated to a temperature of 200°C, and then mixed with IP steam and reacted at the steam reforming rector.
Methanol and steam react at the steam reformer, producing H2 and CO2 as per Equation 2. CO2 is then separated through a physical CO2 capture unit (Selexol) as the gas is pressurized . The separated CO2 is compressed to 138 bar for subsequent sequestration. The H2 is fed to a gas turbine as fuel. The combined cycle is also typical except for H2 rich fuel to the gas turbine and steam turbine with two extractions.
There also are two sources for methanol heating and reforming: IP steam and LP steam. There are two streams of IP steam to the methanol steam-reforming reactor: one as reaction agent and the other as heating source.
The performance results of the combined-cycle system are presented in Case 4 of Table 2. The net plant efficiency and net output are 49.75 percent HHV and 223.5 MW respectively. Because of CO2 capture, both output and efficiency of Case 4 are lower than Case 2.
3.2 Comparison with post-combustion CO2 capture combined cycle
The conventional combined cycle with post-combustion CO2 capture is shown in Figure 1 (by both solid lines and dash lines). A typical MEA unit is used, which needs a large amount of LP steam for solvent regeneration [11, 12]. The steam is extracted from the LP turbine.
The performance results of the post-combustion CO2 capture combined cycle are shown in Case 3 of Table 2. The net plant efficiency and net output are 42.63 percent HHV and 231.8 MW respectively.
Compared to Case 3, the efficiency of Case 4 is about 7.12 percent higher, resulting from a 17.4 percent lower fuel input and only 3.6 percent lower net power output. There are two main reasons:
- In Case 4, a large quantity of low grade heat by IP and LP steam (15 percent fuel energy) is upgraded to the higher energy level through methanol heating and conversion;
- Case 3 needs a large quantity of LP steam (74 percent IP steam turbine flow) for solvent regeneration at the reboiler of the MEA unit, but Case 4 does not consume any steam for CO2 capture.